With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric vehicles and hybrid vehicles have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries used in large size applications which have excellent storage characteristics. Under these circumstances, the high-energy lithium ion secondary batteries having advantages such as a high discharge voltage and a large discharge capacity have been noticed. In particular, in order to apply the lithium ion secondary batteries to electric tools, electric vehicles or the like in which rapid charge/discharge cycle characteristics are needed, it has been required that the lithium ion secondary batteries exhibit excellent rate characteristics.
Hitherto, as a positive electrode (cathode) active substance useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiCoO2, LiNi1/3Co1/3Mn1/3O2 and LiCo1-x-zNixAlzO2 having a rock-salt type structure, or the like. On the other hand, in order to meet the recent need to enhance an energy density of the secondary batteries, there is an increasing demand for those secondary batteries capable of operating at a higher voltage as a single cell.
However, the secondary batteries operating at a high voltage have posed such a problem that an electrolyte used therein tends to suffer from accelerated deterioration, so that gasses tend to be generated due to the deteriorated electrolyte.
In order to suppress generation of the gasses, the positive electrode active substance, electrolyte, etc., have been improved. In addition, it has been further demanded to improve the carbon black used together with the positive electrode active substance.
As the carbon black, there are known carbon black using two kinds of acetylene blacks for the purpose of exhibiting features inherent to the carbon black, i.e., enhancing an output and a service life of batteries (cited Patent Literature 1), and carbon black using two kinds of carbonaceous materials for the purpose of enhancing characteristics of batteries when used under the aforementioned high voltage conditions (cited Patent Literatures 2 and 3). In addition, it is also known to improve a non-aqueous solvent used in lithium ion secondary batteries (cited Patent Literature 4).
However, at present, in the secondary batteries of a high-voltage type having an operating voltage of not less than 4.5 V on the basis of Li, there is a strong demand for a positive electrode mixture capable of conducting stable charging and discharging with a less amount of gasses generated therefrom. However, any materials capable of satisfying the necessary and sufficient conditions have not been obtained yet.
That is, even the technologies described in the aforementioned Patent Literatures 1 to 4 have failed to improve high-temperature characteristics of the secondary batteries to a sufficient extent.
In the Patent Literature 1, there are present no examples comprehensively describing all of a density, impurities, crystallographic parameters of carbon black as main causes of side reactions owing to the carbon black which occur at an interface between a positive electrode and an electrolyte solution in the range of not less than 4.5 V on the basis of Li.
In the Patent Literatures 2 and 3, as a conductive agent, carbon black is used in combination with black lead, hardly-graphitizable carbon or the like. However, in these Patent Literatures, there is no description concerning use of the carbon black only as the conductive agent.
Further, in the conventional arts, there are described merely the technologies for suppressing deterioration in cycle characteristics of batteries which is caused by expansion and contraction of carbon black owing to anion intercalation in the carbon black as well as reduction in density of a positive electrode mixture in association therewith.